Tempstar burner will not stay on


Old 01-13-16, 01:19 PM
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Tempstar burner will not stay on

I have a Tempstar 90+, 2 stage, variable speed 9MPV125 unit that will go through the ignition and rollout sequence but after about 1 1/2 minutes, the burner shuts down, the valve gives 4 flashes, circulating blower continues to run, and after about 2 minutes, the whole sequence starts over.
When this started, it would give 3 flashes, indicating an issue with the pressure switches not closing. To narrow down the problem, I did the following:
1. Disconnected the inlet air line at the furnace to eliminate a blocked air intake.
2. Made sure the pressure switch sensing lines, nipples and condensate drain were clear.
3. Removed the heat exchanger cover and blew out the exchangers. All I got were a few bumblebees.
4. Checked and rechecked the operation of the pressure switches and they are doing what they are supposed to do, open when the inducer is off and closed when it is operating. I even rigged up some bypass switches to be sure they aren't opening during operation and that didn't help.
5. Because of a faulty diagnosis last year by a professional, I have a spare circuit board(once I bought it for the technician since he didn't have time to go get it, it couldn't be returned so it was mine) that I installed with no change.
I know the 4 flashes means that either the rollout switch or the main limit is opening. I checked the two rollout switches and the main limit and they are closed when the system is down. However, I don't understand how the rollout system works because I would think that if these are all wired in series, one of them opening would break that part of the circuit.
I should also mention that at times, there have been 8 flashes from the valve indicating that the high pressure switch is closed when it should be open.
Any and all thoughts are appreciated.
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Old 01-13-16, 01:45 PM
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When was the last time you changed your filter? Try running it without the filter. Reduced house air flow over the exchanger can cause an overheat problem.
Old 01-13-16, 02:13 PM
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Ah, geez. That was the problem. That filter was only in there for about 3 weeks when this started but it was filthy when I took it out. We had the ducts cleaned last year so they should be fairly clean.
I just realized that the filter I pulled out was a somewhat lower rated filter than the 1200 or 1500 rated ones I normally install. Could this one have passed more air and stirred things up enough to clog the filter?
Old 01-14-16, 12:40 PM
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Personally I favor cheap filters, which prevent dust from accumulating on the fan and motor but don't impair the air flow much.

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